Regulating circuit for a pulsed neutron source

ABSTRACT

A pulsed neutron system has an accelerator tube having a target, an ionizing section and a replenisher for supplying accelerator gas. The power supplied to the replenisher is controlled to maintain a constant ionizing pulse time duration. A comparator compares the ionization pulse time duration to the time duration of a reference pulse which is produced each time the accelerator tube is pulsed. The comparator produces a pulse output if the ionization pulse is shorter than the reference pulse. This pulse output is applied to an operational amplifier. Also applied to the operational amplifier is a pulse produced each time the accelerator tube is ionized. The operational amplifier produces a control signal which operates a stepping motor in either a forward or reverse direction. The stepping motor positions a variable autotransformer which increments the voltage applied to the replenisher. The voltage (power) applied to the replenisher is continuously adjusted to maintain a constant ionization pulse width.

United States Patent 91 Dennis 1 March 6, 1973 REGULATING CIRCUIT FOR A PULSED NEUTRON SOURCE [75] Inventor: Charles L. Dennis, Desoto, Tex.

[73] Assignee: Mobile OilCorporation, New York,

[22] Filed: Jan. 29, 1971 [21] Appl. No.: 110,934

[52] US. Cl ..250/84.5, 313/61 S [51] Int. Cl. ..G2lg 3/00 [58] Field of Search ..250/84.5, 83.1; 313/61 S [5 6] References Cited UNITED STATES PATENTS 3,034,008 5/1962 Soloway ..250/83.l X

Primary Examiner-James W. Lawrence Assistant ExaminerDavis L. Willis Attorney-William J. Scherback and Frederick E. Dumoulin [5 7] ABSTRACT A pulsed neutron system has an accelerator tube having a target, an ionizing section and a replenisher for supplying accelerator gas. The power supplied to the replenisher is controlled to maintain a constant ionizing pulse time duration. A comparator compares the ionization pulse time duration to the time duration of a reference pulse which is produced each time the accelerator tube is pulsed. The comparator produces a pulse output if the ionization pulse is shorter than the reference pulse. This pulse output is applied to an operational amplifier. Also applied to the operational amplifier is a pulse produced each time the accelerator tube is ionized. The operational amplifier produces a control signal which operates a stepping motor in either a forward or reverse direction. The stepping motor positions a variable autotransformer which increments the voltage applied to the replenisher. The voltage (power) applied to the replenisher is continuously adjusted to maintain a constant ionization pulse width.

6 Claims, 4 Drawing Figures TRIGGER PULSE GEN.

END OF SHORT ION PULSE PATENTEU 51973 3. 7 1 9 82 7 SHEET 10F 2 TRIGGER PULSE GEN.

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SHEET 2 BF 2 TTE. 2'A

REGULATING CIRCUIT FOR A PULSED I NEUTRON SOURCE BACKGROUND OF THE INVENTION neutron detector are operated in a borehole which 1 passes through formations of interest. This formation is periodically irradiated with bursts of fast neutrons. The neutrons resulting from the irradiation are detected and the quantity detected is recorded. Copending application Ser. No. 45,458 filed June ll, 1970, Givens, Caldwell, and Mills, Jr. describes such a system. I

A pulsed neutron generator for such a system commonly takes the form of a three-element linear accelerator tube. It includes a replenisher element which is electrically heated to boil off deuterium gas adsorbed by the filament. The deuterium molecules are ionized by an ionizing section which commonly includes plates to which a positive ionization pulse is applied. The deuterium ions are then accelerated and bombard a target which includes tritium molecules. The bombardment of the deuterium ions on the tritium molecules yields helium plus a supply of neutrons This reaction Commonly, such a tube can be operated to produce neutrons per pulse at two pulses per second. The neutrons commonly have an energy of approximately 14 m.e.v. One commercially available tube which is capable of such operation is the Kaman Nuclear Model A-80l Neutron Generator.

In operating such a tube it is important that the power supplied to the replenisher be correctly adjusted so that the proper amount of accelerator gas, deuterium as described above, boils off the replenisher element. If the replenisher is overheated, too much accelerator gas boils off. In this case, ion recombination takes place in the tube. Also, arcing in the tube shortens the tube life and neutron output falls off. If too little power is supplied to the replenisher, there is not enough accelerator gas available in the tube to provide a good neutron output.

The adjustment of the power supplied to the replenisher is complicated by the fact that the characteristics of the tube change as the tube ages. That is, after the tube has been in use, a greater amount of power must be supplied to the replenisher to boil off the same amount of accelerator gas. In tubes such as the aforementioned Kaman tube, a semiautomatic adjustment is provided for the replenisher supplyvoltage.

While semi-automatic control of the replenisher is acceptable for use in a laboratory, for example, it is not suitable for field use. In particular, it is not suitable for use in a well logging system wherein (a) the tube is encased in a logging tool and is inaccessible to the operator during the time that it is in use, and (b) it must always provide optimum neutron output during the normal lifespan of the tube.

SUMMARY OF THE INVENTION In accordance with an important aspect of this invention, the power supply to the replenisher element in a linear accelerator tube is automatically controlled. Further, in accordance with this invention the operating parameter which is monitored is the time duration of the ionization pulse. If too much accelerator gas is boiled off the replenisher, the time duration of the ionization pulse is shorter than normal. If too little accelerator gas is boiled off, the time duration is longer than normal. By comparing the time duration of the ionization pulse to a reference pulse, a control signal is generated which continuously adjusts the power supplied to the replenisher to maintain the ionization time substantially constant.

In one specific embodiment of the invention, the regulator includes a comparator to which the ionization 'pulse is applied. Each time an ionization" pulse is generated, a reference pulse of constant time duration is generated and applied to the comparator. If the ionization pulse is shorter in duration than the reference pulse, a pulse output is produced. This pulse output is applied to an operational amplifier. The other input to this operational amplifier is a pulse produced each time the accelerator tube is ionized. If both pulses are present at the inputs of the operational amplifier, a control signal is applied to a motor which steps the motor in one direction. If only one pulse is present, the motor is stepped in the other direction. Each time the accelerator tube is ionized, the motor is advanced in one direction or the other. This motor increments a variable auto-transformer which supplies power to the replenisher. In this manner, the replenisher power is continuously adjusted to supply the correct amount of accelerator gas to the tube.

In accordance with another aspect of this invention, the regulator circuit produces accurately timed high voltage pulses for the ionizing section and the target of the tube.

The foregoing and other objects, features and advantagesof the invention will be better understood from the following more detailed description, appended claims and drawings.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block diagram of the regulator system of this invention;

FIG. 2A shows an ionization pulse which is of too short duration;

FIG. 2B shows an ionization pulse of normal duration; and

FIG. 2C shows an ionization pulse having a time duration which is too long.

DESCRIPTION OF A PARTICULAR EMBODIMENT Referring to FIG. 1, the pulsed neutron system includes an accelerator tube 11 having a target 12, an ionization section including plates 13 and 14, and a replenisher 15 for supplying accelerator gas.

Deuterium ions emitted by the replenisher are ionized by a +5 kv ionization pulse applied across the plates 13 and 14. The deuterium ions are accelerated toward the target 12 by a l20 kv pulse applied to the target. The generation of the +5 kv ionization pulse and the generation of the -l20 kv pulse applied to the target 12 will first be described.

Energy for the production of these pulses is stored in the storage capacitor 16. This energy is generated by a 110-volt, 400-cycle source which is connected to the primary of the transformer 17. The secondary winding of transformer 17 produces a voltage of approximately three to 5 kilovolts. This voltage is rectified as indicated by the halfwave rectifying diode 18. The rectified voltage is applied to the storage capacitor 16 which is periodically discharged by a switch. The switch includes the xenon-filled triggerable spark gap 19. A trigger pulse generator 20 generates the trigger pulses which fire the spark gap 19 at periodic intervals, the interval being two pulses per second in one embodiment of the invention.

Each time the spark gap 19 is triggered, the energy stored in capacitor 16 is applied to the primaries of transformers 21 and 22. The secondary winding of the transformer 21 produces a positive S-kilovolt pulse which is applied to the plates 13 and 14 to ionize the accelerating gas in the tube. These positive ions are then accelerated toward the target 12 by a l20 kv pulse applied to the target. Since the ionization process requires a finite amount of time whereas the acceleration is relatively instantaneous, the acceleration pulse is delayed from the ionizing pulse. A delay line 23 provides approximately a 7 microsecond delay for the acceleration pulse relative to the ionization pulse. The delay line 23 also acts as a tuned circuit with capacitor 24. This circuit is tuned to most efficiently transfer energy from the storage capacitor 16 to the target 12 of the tube.

Before describing the regulator circuit for the replenisher, the importance of providing regulation will be described with reference to FIGS. 2A-2C. FIG. 2B shows the normal width ionization pulse appearing across the plates of the ionization section. For the Kaman tube previously referred to, the suggested time duration of this pulse is approximately three microseconds. If the replenisher boils off an overabundance of accelerator gas, then the ionization pulse will be more quickly loaded to the ionization potential. This is shown in FIG. 2A wherein the ionization pulse prematurely drops to the ionization potential 25. On the other hand, if there is not sufficient accelerator gas supplied from the replenisher, a longer time is required for ionization. This is shown in FIG. 2C wherein the ionization pulse has a time duration which is longer than is desired. It is desired to regulate the power supplied to the replenisher so that the ionization pulse is always of approximately the nominal time duration, in this case approximately 3 microseconds. The regulator circuit for accomplishing this includes an attenuator 26 for attenuating the ionization pulse to a level which can be accommodated by digital logic circuits. That is, the attenuator may be through of as generating an ionization pulse having the same time duration as the ionization time of the accelerator gas in the tube 11.

In FIGS. 2A-2C note that there is noise present. It is desired to measure the time duration of the ionization pulse above the level of this noise. In order to do this, a Schmidt trigger 27 is provided. The Schmidt trigger 27 comprises two Nand gates 28 and 29 which produce a square pulse having a time duration which is the same as the time duration of the ionization pulse above the noise. The negative-going pulse which is the output of Nand circuit 28 is applied to a comparator which includes the Nand gates 30 and 31. A reference pulse is produced by the one-shot multivibrator 32 each time that the accelerator tube 11 is fired. If the ionization pulse is of shorter duration than the I i-microsecond reference pulse, the Nand gate 30 (comparator) and the Nand gate 31 (inverter) produce a positive-going pulse. This pulse is used to trigger a pulse generator 33 which generates a positive-going pulse of accurately controlled width. In the embodiment under consideration, the pulse width is 0.1 second. This 0.1 second pulse is applied to one input of the operational amplifier 34. The operational amplifier 34 producesa control signal which is applied to a control element. In this example, the control element includes a permanent magnet D.C. motor 35 which drives the variable autotransformer 36. The variable autotransformer 36 applies power to the replenisher'15. r

Another input to operational amplifier 34 is from a pulse generator 37 which is triggered each time an ionization pulse is produced. The pulse generator 33 has the capability for trimming the pulse width so that the pulse output from the generator 33, if one is present, coincides exactly in time (0.1 second) with the pulse output from the generator 37. The output of pulse generator 33 is applied to the input which has a positive X 2 amplification. The output of pulse generator 37 is applied to an input of operational amplifier 34 which has a negative X l amplification. The result is that the control signal produced by the operational amplifier 34 is a positive-going pulse if the time duration of the ionization pulse is too short. Alternatively, the control signal from the operational amplifier 34 is a negative-going pulse if the time duration of the ionization pulse is too long. Each time the accelerator tube 11 is fired, the operational amplifier 34 produces a pulse which increments the motor 35 in one direction or another. Because of this, there is continuous hunting of the regulator system around the null point. However, this minor disadvantage of continuous hunting is more than offset by the simplicity of the control system which is a particular requirement for the pulsed neutron system being described.

In one embodiment of the invention, a current of approximately 2 amperes is applied to the replenisher element 15. The motor 35 moves the variable autotransformer 36 in increments which change the current by approximately 10 milliamperes. This provides an acceptable regulation for the system described.

The following are typical circuit elements used in one embodiment of the invention. They are given by way of example only and are not to be considered to be limiting of the invention. The Nand gates 28, 29, 30, and 31, the one-shot multivibrator 32, and the pulse generators (also one-shot multivibrators) 33 and 37 are integrated circuits manufactured under the trade name High Threshold Logic by MOtorola. The xenon-filled trigger volt spark gap is manufactured by e.g. The stepping motor 35 is a Globe permanent magnet D.C. motor. The variable autotransformer 36 is a Superior Model IHSOIOK.

As previously mentioned, the Kaman Model A-801 linear accelerator is suitable for use as the tube 11. Transformer 17, 21, and 22 may be those manufactured by Geotronics, Power Designs, and Kaman Nuclear, respectively. A storage capacitor suitable for use as the capacitor 16 is manufactured by Maxwell Labs.

Briefly, the operation of the system is as follows. The trigger pulse generator 20 produces repetitive pulses of approximately tow per second. Each pulse triggers the spark gap 19 which discharges the capacitor 16 into the transformers 21 and 22. The aforementioned 21 produces an ionization pulse of approximately 5 kv which, applied to the ionization section including plates 13 and 14, ionizes deuterium molecules boiled off of the replenisher 15.

A short time later the transformer 22 produces a l20 kv acceleration pulse which is applied to the target 12. Bombardment of the target 12 by deuterium atoms initiates a reaction which liberates a burst of neutrons.

The regulation of the power supplied to the replenisher is provided by a control circuit which compares the time duration of the ionization pulse with the time duration of a reference pulse produced by the one-shot multivibrator 32. The comparator, including NAND circuits 30 and 31, produces a pulse if the ionization pulse is shorter in duration than the reference pulse; otherwise no pulse is produced by the comparator. When the comparator produces a pulse, the pulse generator 33 applies a pulse input to an input of operational amplifier 34 which amplifies that pulse by two times. Since the pulse applied by pulse generator 37 is amplified only by unity (-1), the control signal produced by operational amplifier 34 is a positive pulse. This increments the motor 35 in a direction which decreases the power supplied to the replenisher 15.

Conversely, if the time duration of the ionization pulse is not shorter than the time duration of the reference pulse, only the pulse generator 37 applies a pulse to the operational amplifier. This produces a negative-going pulse as the control signal. This negative-going pulse increments the motor 35 in a direction which increases the power supply to the replenisher 15.

When the time duration of the ionization pulse is nominal, that is, it approximates the time duration of the reference pulse, the motor 35 will be alternately stepped in one direction and then another on successive bursts ofthe accelerator tube 11.

While a particular embodiment of the invention has been shown and described, various modifications will be apparent. The appended claims are, therefore, intended to cover any such modifications.

What is claimed is:

1. A pulsed neutron system comprising:

an accelerator tube having a target, an ionization section and a replenisher for supplying accelerator gas,

a control element for varying the power supplied to said replenisher in response to a control signal,

means for generating an ionization pulse having the same time duration as the ionization time of the accelerator gas, and

a comparator, said ionization pulse being applied to said comparator, said comparator producing a control signal representing the deviation of the time duration of said ionization pulse from a reference, said control signal being applied to said control elementto control the power su plied to said replenisher at a level which main arm the ionization time of each neutron pulse substantially constant. 2. The system recited in claim 1 wherein said comparator comprises:

means for producing a reference pulse, having a nominal time duration, each time said accelerator gas is ionized, and

means for comparing the time duration of said reference pulse to the time duration of said ioniza tion pulse to produce a control pulse when one time duration is shorter than the other time duration.

3. The system recited in claim 2 wherein said control element comprises a motor which is stepped in one direction by said control pulse to change the power supplied to said replenisher.

4. The system recited in claim 3 wherein said comparator further comprises:

means for producing a pulse which steps said motor in the opposite direction when said one time duration is not shorter than said other time duration.

5. The system recited in claim 1 further comprising:

a storage capacitor,

means for charging said capacitor to a high voltage,

and

switching means for periodically discharging said capacitor to produce a high voltage ionization pulse applied to said ionization section.

6. The system recited in claim 5 further comprising:

delay means responsive to said switching means for producing an accelerating pulse applied to said target, said accelerating pulse being delayed with respect to said ionization pulse. 

1. A pulsed neutron system comprising: an accelerator tube having a target, an ionization section and a replenisher for supplying accelerator gas, a control element for varying the power supplied to said replenisher in response to a control signal, means for generating an ionization pulse having the same time duration as the ionization time of the accelerator gas, and a comparator, said ionization pulse being applied to said comparator, said comparator producing a control signal representing the deviation of the time duration of said ionization pulse from a reference, said control signal being applied to said control element to control the power supplied to said replenisher at a level which maintains the ionization time of each neutron pulse substantially constant.
 1. A pulsed neutron system comprising: an accelerator tube having a target, an ionization section and a replenisher for supplying accelerator gas, a control element for varying the power supplied to said replenisher in response to a control signal, means for generating an ionization pulse having the same time duration as the ionization time of the accelerator gas, and a comparator, said ionization pulse being applied to said comparator, said comparator producing a control signal representing the deviation of the time duration of said ionization pulse from a reference, said control signal being applied to said control element to control the power supplied to said replenisher at a level which maintains the ionization time of each neutron pulse substantially constant.
 2. The system recited in claim 1 wherein said comparator comprises: means for producing a reference pulse, having a nominal time duration, each time said accelerator gas is ionized, and means for comparing the time duration of said reference pulse to the time duration of said ionization pulse to produce a control pulse when one time duration is shorter than the other time duration.
 3. The system recited in claim 2 wherein said control element comprises a motor which is stepped in one direction by said control pulse to change the power supplied to said replenisher.
 4. The system recited in claim 3 wherein said comparator further comprises: means for producing a pulse which steps said motor in the opposite direction when said one time duration is not shorter than said other time duration.
 5. The system recited in claim 1 further comprising: a storage capacitor, means for charging said capacitor to a high voltage, and switching means for periodically discharging said capacitor to produce a high voltage ionization pulse applied to said ionization section. 